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What does "crankshaft imbalance factor" mean?

Ford 302 v8 engine production crankshafts apparently come in two types, one (the older models, 1966 to about 1980) come with a 26 ounce imbalance factor, and the other (the newer models, appx 1980 to 1985) come with a 50 ounce imbalance factor. I think the unit for imbalance factor is once, maybe it is gram. Anyway, what’s that imbalance factor all about? Just curious, if they know the crank isn’t properly balanced, why don’t they just balance it at the factory so the imbalance factor is always zero for a production crank? I’m assuming there’s a reason they do it that way, but why introduce that compatibility complication?.

It’s really complicated topic and far too involved to type on this little phone keyboard so I went to find something already explaining it and found this. Really good explanation imo-

This link discusses it.

Thanks, good info in the links. The common unit for the imbalance factor appears to be “ounce-inch”. That makes sense, to balance something that rotates you need the right weight at the right distance from the center of rotation, so the unit pretty much has to be the product of mass times distance.

Yup. Any time you’re dealing with rotational inertia, distance of the weight from the rotating axis matters.